Aircraft primary structures are predominately made from non-composite metals. However, the aerospace industry has been increasingly using light weight, advanced composite materials in place of metals to produce primary structures because of the high specific strength of advanced composites materials. Nevertheless, advanced composite materials have not entirely replaced metals in primary structures because advanced composites are more sensitive to damage, have lower bearing strength, and are more susceptible to fastener failure than metals.
Several improved composites have been designed, including Arall, as disclosed in U.S. Pat. No. 4,500,589, and Glair, as disclosed in U.S. Pat. No. 5,039,571. Disadvantageously, however, the layers of both Arall and Glair have a mismatch of the ratio between their modulus of elasticity and their yield strength.
For example, Arall is a composite of aluminum skins adhesively bonded to a core of Aramid fiber/epoxy composite. The Aramid fiber of Arall has a unidirectional yield strength of about 172,000 psi and a modulus of 12.2.times.10.sup.6 psi, while the aluminum layer has a yield strength of 50,000 psi and modulus of 10.0.times.10.sup.6 psi. Thus, stressing the Aramid fiber layer to its maximum yield strength would stress the aluminum layer to 141,000 psi, which is well above the maximum limit for the aluminum layer. Conversely, stressing the aluminum layer to its maximum yield strength of 50,000 psi stresses the Aramid fiber layer to 61,000 psi, which is well below the maximum limit for the Aramid fiber layer. Thus, the strength of the Aramid fiber layer is underutilized. Similarly, the layers of composite laminates of standard alpha-beta alloys of titanium, such as Ti6Al-4V, and carbon fiber composites have a mismatch of the ratio between their modulus of elasticity and their yield strength.
The aerospace industry has not used the newer beta alloys of titanium, such as TIMETAL.RTM. 15-3 (Ti-15V-3Cr-3Sn-3Al) and TIMETAL.RTM. 21s (Ti-15Mo-3Al-3Nb), for composite laminates even though these beta alloys of titanium have higher strength and a lower modulus of elasticity because commonly used adhesives will not stick adequately to the titanium oxide surface layer present on these alloys. While methods have been developed to bond the standard alpha-beta alloys of titanium to carbon fiber composites, these methods do not work with the beta alloys of titanium.
Therefore, there is a need for improved composite laminates for the primary structures of aircraft which utilize the full strength of each layer. Further, there is a need for a method of preparing these composite laminates.